1. Field of the Invention
The present invention relates generally to memory systems and in particular to memory systems having multiple memory devices and a controller for serial selection of the memory devices.
2. Description of Related Art
Data systems incorporating memory systems having multiple memory devices are well known. By way of example, FIG. 1 depicts a simplified conventional memory system which includes a host device 20, an address decoder 22 and memory devices 24A and 24B. The host device 22 may be a microprocessor and the memory devices 24A and 24B may be separate memory integrated circuits. An address bus 26 is used to provide addresses to an address decoder 22 and to the memory devices 24A and 24B. The address decoder 22 has two outputs connected to enable inputs of the memory devices 24A and 24B. Typically, the most significant bit(s) of the address are provided on the bus 26 to the decoder 22, with the remaining address bits being provided to each of the memory devices.
When the memory is to be accessed, the processor 20 causes the address decoder 22 to decode the most significant bit(s) of the memory address placed on an address bus 26. The decoder 22 will select one of the two memory devices 24A and 24B by generating either signal Sel 0 or Sel 1. The selected memory device will respond to the address presented to it on the address bus and the deselected memory device, which is disabled, will not respond. Although not shown, a data bus is used to transfer data between the memory devices and the processor 20, with only the selected device outputting data to the data bus during memory read operations.
The approach depicted in FIG. 1 is sometimes referred to as radial device selection where each memory device has a separate select input. This approach works well when relatively few memory devices are employed and where access speed, particularly random access speed, is important. However, if a large number of memory devices are used so that large amounts of data can be stored, the requirement of separate select lines for each memory device results in large memory boards and a relatively large pin count for the control logic circuitry. Thus, unless access speed is critical and a large number of memory devices are used, the radial device selection approach of FIG. 1 is not ideal.
FIG. 2 shows an alternative prior art device selection technique, sometimes referred to as serial selection. Again, a host device 28 is used which is connected to several memory devices 30A, 30B and 30C by way of a system bus 32. The memory devices 30A, 30B and 30C are usually implemented as separate integrated circuits. The system bus 32 includes memory address and memory data and various control signals so that each of the memory devices 30A, 30B 30C receives the same addresses, data and other signals. Each memory device is preassigned a unique address so that only one device will be accessed by the host device 28 during a memory operation. Typically, the memory devices 30A, 30B and 30C are assigned addresses by way of jumper or switch settings represented by elements 34A, 34B and 34C.
The jumpers or switch settings represented by elements 34A, 34B and 34C require appropriate hardware which increases costs and utilizes memory board space. In addition, if additional memory devices are to be added to a memory system, a user has to determine an appropriate address for the added devices. This determination requires that a user ascertain what address ranges are not available and which addresses are free to be assigned to the new memory devices. Thus, there is a distinct possibility for error.
The FIG. 2 approach also requires that dedicated pins be provided on each of the integrated circuit memory devices 30A, 30B and 30C to receive the jumper wires or switches for assigning the addresses. These pins increase the pin count for the integrated circuits thereby increasing the cost of the packaging for the devices and increasing the likelihood that there will be mechanical problems and manufacturing errors through soldering and the like. These extra pins are also subject to defects and increase the possibility of damage to the integrated circuits as a result of electrostatic discharge.
There is a need for a memory system which provides the advantages of serial selection techniques, but allows the addition of memory devices without introducing the possibility of user error when such devices are added. Further, there is a need for a system having a reduce pin count. The present invention provides this and other advantages as will be appreciated by those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings.